Over-coating agent for forming fine patterns and a method of forming fine patterns using such agent

ABSTRACT

It is disclosed an over-coating agent for forming fine-line patterns which is applied to cover a substrate having thereon photoresist patterns and allowed to shrink under heat so that the spacing between adjacent photoresist patterns is lessened, further characterized by comprising a water-soluble polymer which contains at least methacrylic acid and/or methyl methacrylate as the constitutive monomer thereof. Also disclosed is a method of forming fine-line patterns using the over-coating agent. The advantages of the invention are that the exposure margin is large, that the dimension control of photoresist patterns can be reflected on the dimension controllability in forming fine-line patterns, that the dimension control and planning of forming fine trace patterns after treatment for thermal shrinkage can be attained with ease in the stage of photoresist patterning, that the original photoresist pattern profile can be kept as such and the top of the photoresist pattern is not rounded after thermal shrinkage, that the degree of thermal shrinkage of the over-coating agent is large and thus the agent is effective in forming fine-line patterns.

BACKGROUND OF THE INVETNION

1. Field of the Invention

This invention relates to an over-coating agent for forming finepatterns in the field of photolithographic technology and a method offorming fine-line patterns using such agent. More particularly, theinvention relates to an over-coating agent for forming or definingfine-line patterns, such as hole patterns and trench patterns, that canmeet today's requirements for higher packing densities and smaller sizesof semiconductor devices.

2. Description of the Related Art

In the manufacture of electronic components such as semiconductordevices and liquid-crystal devices, there is employed thephotolithographic technology which, in order to perform a treatment suchas etching on the substrate, first forms a film (photoresist layer) overthe substrate using a so-called radiation-sensitive photoresist which issensitive to activating radiations, then performs exposure of the filmby selective illumination with an activating radiation, performsdevelopment to dissolve away the photoresist layer selectively to forman image pattern (photoresist pattern), and forms a variety of patternsincluding contact providing patterns such as a hole pattern and a trenchpattern using the photoresist pattern as a protective layer (maskpattern).

With the recent increase in the need for higher packing densities andsmaller sizes of semiconductor devices, increasing efforts are beingmade to form sufficiently fine-line patterns and submicron-electronicfabrication capable of forming patterns with linewidths of no more than0.20 μm is currently required. As for the activating light raysnecessary in the formation of mask patterns, short-wavelength radiationssuch as KrF, ArF and F₂ excimer laser beams and electron beams areemployed. Further, active R&D efforts are being made to find photoresistmaterials as mask pattern formers that have physical properties adaptedto those short-wavelength radiations.

In addition to those approaches for realizing submicron-electronicfabrication which are based on photoresist materials, active R&D effortsare also being made on the basis of pattern forming method with a viewto finding a technology that can provide higher resolutions than thosepossessed by photoresist materials.

For example, JP-5-166717A discloses a method of forming fine patternswhich comprises the steps of defining patterns (=photoresist-uncoveredpatterns) in a pattern-forming resist on a substrate, then coating overentirely the substrate with a mixing generating resist that is to bemixed with said pattern-forming resist, baking the assembly to form amixing layer on both sidewalls and the top of the pattern-formingresist, and removing the non-mixing portions of said mixing generatingresist such that the feature size of the photoresist-uncovered patternis reduced by an amount comparable to the dimension of said mixinglayer. JP-5-241348 discloses a pattern forming method comprising thesteps of depositing a resin, which becomes insoluble in the presence ofan acid, on a substrate having formed thereon a resist patterncontaining an acid generator, heat treating the assembly so that theacid is diffused from the resist pattern into said resin insoluble inthe presence of an acid to form a given thickness of insolubilizedportion of the resist near the interface between the resin and theresist pattern, and developing the resist to remove the resin portionthrough which no acid has been diffused, thereby ensuring that thefeature size of the pattern is reduced by an amount comparable to thedimension of said given thickness.

However, in these methods, it is difficult to control the thickness oflayers to be formed on the sidewalls of resist patterns. In addition,the in-plane heat dependency of wafers is as great as ten-odd nanometersper degree Celsius, so it is extremely difficult to keep the in-planeuniformity of wafers by means of the heater employed in currentfabrication of semiconductor devices and this leads to the problem ofoccurrence of significant variations in pattern dimensions.

Another approach known to be capable of reducing pattern dimensions isby fluidizing resist patterns through heat treatment and the like. Forexample, JP-1-307228A discloses a method comprising the steps of forminga resist pattern on a substrate and applying heat treatment to deformthe cross-sectional shape of the resist pattern, thereby defining a finepattern. In addition, JP-4-364021A discloses a method comprising thesteps of forming a resist pattern and heating it to fluidize the resistpattern, thereby changing the dimensions of its resist pattern to formor define a fine-line pattern.

In these methods, the wafer's in-plane heat dependency is only a fewnanometers per degree Celsius and is not very problematic. On the otherhand, it is difficult to control the resist deformation and fluidizingon account of heat treatment, so it is not easy to provide a uniformresist pattern in a wafer's plane.

An evolved version of those methods is disclosed in JP-7-45510A and itcomprises the steps of forming a resist pattern on a substrate, forminga stopper resin on the substrate to prevent excessive thermal fluidizingof the resist pattern, then applying heat treatment to fluidize theresist so as to change the dimensions of its pattern, and thereafterremoving the stopper resin to form or define a fine-line pattern. As thestopper resin, a water-soluble resin, specifically, polyvinyl alcohol isemployed singly. However, polyvinyl alcohol alone is not highly solublein water and cannot be readily removed completely by washing with water,introducing difficulty in forming a pattern of good profile. The patternformed is not completely satisfactory in terms of stability over time.In addition, polyvinyl alcohol cannot be applied efficiently by coating.Because of these and other problems, the method disclosed in JP-7-45510has yet to be adopted commercially.

For solving these prior-art problems, the present applicant has proposeda technique directed to an over-coating agent for forming fine patternsand to a method of forming fine patterns in JP 2003-084459A, JP2003-084460A, JP 2003-107752A, JP 2003-142381A, JP 2003-195527A, and JP2003-202679A, etc. The technique shown in these patent publications hasmade it possible to form fine-line patterns that satisfy patterndimension controllability, good profile and other necessary propertiesfor semiconductor devices.

In the technique of forming fine-line patterns using the aboveover-coating agent for forming fine patterns, a photoresist layer isfirst formed on a substrate and this is exposed to light and developedto form a photoresist pattern (mask pattern). Next, the over-coatingagent for forming fine patterns is applied to cover the entire surfaceof the substrate, and then this is heated, whereby the width of thephotoresist pattern lines is enlarged by utilizing the thermal shrinkingeffect of the over-coating agent for forming fine patterns, and, as aresult, the distance between the adjacent photoresist pattern lines isthereby narrowed and the width of the pattern line (of various patternssuch as hole pattern, and trench pattern) to be determined by thedistance between the photoresist pattern lines is also narrowed to givefiner trace patterns.

The above-mentioned forming fine-line pattern process undergoes theinfluence of pattern dimension control in two stages: that is, aphotoresist patterning stage (first stage) and a thermal shrinking stageof the over-coating agent for forming fine patterns (second stage). Inthe process comprising said two stages, when photoresist patterning isperformed by increasing the luminous exposure of light to which aphotoresist is exposed in the first stage, the degree of thermalshrinkage of the over-coating agent in the second stage is apt to belarger than the expected degree thereof and, as a result, it is oftendifficult to anticipate the dimension controllability in formingfine-line patterns in the process.

In that situation, it is desirable that the degree of thermal shrinkageof the over-coating agent could be kept constant even when the luminousexposure is varied relative to CD (critical dimension) of thephotoresist pattern just after development.

In addition, it is also desirable that, even when various patternsdiffering in the pattern dimension and the line-to-line distance existon one substrate, all the patterns could enjoy the same degree ofthermal shrinkage.

In view of the shape of photoresist patterns, even when photoresistpatterns having a good rectangular cross-sectional profile could beformed in the photoresist patterning stage (first stage), there mayoccur in the second stage a problematic phenomenon that the top of thephotoresist pattern may be rounded owing to the thermal shrinkage of theover-coating agent applied thereto for forming fine patterns. Inparticular, when an ArF photoresist is used for forming finer linepatterns, the top of the photoresist pattern could not keep the originalrectangular profile thereof but is often rounded in the thermalshrinkage step for the over-coating agent applied onto the photoresistpattern.

Accordingly, an over-coating agent for forming fine patterns has beenstudied and developed, which ensures a high degree of thermal shrinkageand a large exposure margin and enables pattern dimension control whilekeeping a good photoresist pattern profile as such.

JP 2001-281886A discloses a method comprising the steps of covering asurface of a resist pattern with an acidic film made of a resist patternsize reducing material containing a water-soluble resin, rendering thesurface layer of the resist pattern alkali-soluble, then removing saidsurface layer and the acidic film with an alkaline solution to reducethe feature size of the resist pattern. JP-2002-184673A discloses amethod comprising the steps of forming a resist pattern on a substrate,then forming a film containing a water-soluble film forming component onsaid resist pattern, heat treating said resist pattern and film, andimmersing the assembly in an aqueous solution of tetramethylammoniumhydroxide, thereby forming a fine-line resist pattern without involvinga dry etching step. However, both methods are simply directed toreducing the size of resist trace patterns themselves and therefore aretotally different from the present invention in object.

SUMMARY OF THE INVENTION

The present invention has been accomplished in order to solve theaforementioned problems of the prior art and has as an object providingan over-coating agent for forming fine line patterns whose advantagesare that, in forming fine line patterns utilizing the over-coatingagent, the exposure margin is large, the photoresist pattern dimensioncontrol can be reflected on the fine-line pattern dimension control, thephotoresist pattern profile is kept rectangular after a step of thermalshrinkage while its top is prevented from being rounded, and the degreeof thermal shrinkage of the over-coating agent is large relative to theheating temperature at which the agent is heated.

Another object of the invention is to provide a method of forming finetrace patterns using the over-coating agent.

In order to attain the first object, the present invention provides anover-coating agent for forming fine patterns which is applied to cover asubstrate having photoresist patterns thereon and allowed to shrinkunder heat so that the spacing between adjacent photoresist patterns islessened, further characterized by comprising a water-soluble polymerwhich contains at least methacrylic acid and/or methyl methacrylate asthe constitutive monomer thereof.

In order to attain the second object, the present invention provides amethod of forming fine patterns comprising the steps of covering asubstrate having thereon photoresist patterns with the above-describedover-coating agent for forming fine patterns, then applying heattreatment to shrink the applied over-coating agent under the action ofheat so that the spacing between adjacent photoresist patterns islessened, and subsequently removing the applied film of the over-coatingagent substantially completely.

In a preferred embodiment, the heat treatment is performed by heatingthe assembly at a temperature that does not cause thermal fluidizing ofthe photoresist patterns on the substrate.

DETAILED DESCRIPTION OF THE INVENTION

The over-coating agent of the invention for forming fine features ofpatterns is applied to cover a substrate, having photoresist patterns(mask patterns) thereon, including patterns typified by hole patterns ortrench patterns, each of these patterns are defined by spacing betweenadjacent photoresist patterns (mask patterns). Upon heating, the appliedfilm of over-coating agent shrinks to increase the width of each of thephotoresist patterns, thereby narrowing or lessening adjacent holepatterns or trench patterns as defined by spacing between thephotoresist patterns and, thereafter, the applied film is removedsubstantially completely to form or define fine patterns.

The phrase “removing the applied film substantially completely” as usedherein means that after lessening the spacing between adjacentphotoresist patterns by the heat shrinking action of the appliedover-coating agent, said film is removed in such a way that nosignificant thickness of the over-coating agent will remain at theinterface with the photoresist patterns. Therefore, the presentinvention does not include methods in which a certain thickness of theover-coating agent is left intact near the interface with thephotoresist pattern so that the feature size of the pattern is reducedby an amount corresponding to the residual thickness of the over-coatingagent.

The over-coating agent for forming fine patterns of the inventioncomprises a water-soluble polymer that contains at least methacrylicacid and/or methyl methacrylate as the constitutive monomer thereof.

Since the water-soluble polymer in the agent contains methacrylic acidand/or methyl methacrylate as the constitutive monomer thereof, it ispossible to significantly improve the degree of thermal shrinkage of theover-coating agent to thereby reduce the line-to-line distance of thephotoresist pattern during the thermal shrinkage of the agent under heattreatment while keeping the photoresist pattern profile as such.

For preferred embodiments of the over-coating agent of the invention,the water-soluble polymer is preferably any of the following embodiments(i) to (iv), to which, however, the invention should not be limited.

(i) The water-soluble polymer is a copolymer of methacrylic acid and/ormethyl methacrylate with at least one monomer selected from thoseconstituting alkylene glycol-based polymers, cellulosic derivatives,vinylic polymers, acrylic polymers, urea-based polymers, epoxy polymers,amide-based polymers and melamine-based polymers (in which the monomersto constitute acrylic polymers do not include methacrylic acid andmethyl methacrylate).

The monomers to constitute alkylene glycol-based polymers include, forexample, ethylene glycol and propylene glycol.

The monomers to constitute cellulosic derivatives include, for example,hydroxypropylmethyl cellulose phthalate, hydroxypropylmethyl celluloseacetate phthalate, hydroxypropylmethyl cellulose hexahydrophthalate,hydroxypropylmethyl cellulose acetate succinate, hydroxypropylmethylcellulose, hydroxypropyl cellulose, hydroxyethyl cellulose, celluloseacetate hexahydrophthalate, carboxymethyl cellulose, ethyl cellulose andmethyl cellulose.

The monomers to constitute vinylic polymers include, for example,N-vinylpyrrolidone, vinylimidazolidinone and vinyl acetate.

The monomers to constitute acrylic polymers (excluding methacrylic acidand methyl methacrylate) include, for example, acrylic acid, methylacrylate, N,N-dimethylacrylamide, N,N-dimethylaminopropylmethacrylamide,N,N-dimethylaminopropylacrylamide, N-methylacrylamide,diacetonacrylamide, N,N-dimethylaminoethyl methacrylate,N,N-diethylaminoethyl methacrylate, N,N-dimethylaminoethyl acrylate andacryloylmorpholine.

The monomers to constitute urea-based polymers include, for example,methylolated urea, dimethylolated urea and ethylene-urea.

The monomers to constitute melamine-based polymers include, for example,methoxymethylated melamine, methoxymethylated isobutoxymethylatedmelamine and methoxyethylated melamine.

Of monomers constituting epoxy polymers and amide-based polymers,water-soluble monomers are usable herein.

In the copolymer of methacrylic acid and/or methyl methacrylate with themonomer to constitute the above-mentioned polymers, the monomerpreferably accounts for 60-99 mass %, more preferably 80-99 mass % ofmethacrylic acid and/or methyl methacrylate.

(ii) The water-soluble polymer is a copolymer or a mixed resin ofpolymethacrylic acid and/or polymethyl methacrylate with at least onepolymer selected from alkylene glycol-based polymers, cellulosicderivatives, vinylic polymers, acrylic polymers (excludingpolymethacrylic acid and polymethyl methacrylate), urea-based polymers,epoxy polymers, amide-based polymers and melamine-based polymers.

Preferably, the alkylene glycol-based polymers, the cellulosicderivatives, the vinylic polymers, the acrylic polymers (excludingpolymethacrylic acid and polymethyl methacrylate), the urea-basedpolymers, the epoxy polymers, the amide-based polymers and themelamine-based polymers are those comprising any of the above-mentionedconstitutive monomers.

Preferably, the content of polymethacrylic acid and/or polymethylmethacrylate in the water-soluble polymer is 60-99 mass %, morepreferably 80-99 mass %.

(iii) The water-soluble polymer is a copolymer of methacrylic acidand/or methyl methacrylate, acrylic acid and/or methyl acrylate, and atleast one monomer selected from those constituting alkylene glycol-basedpolymers, cellulosic derivatives, vinylic polymers, acrylic polymers,urea-based polymers, epoxy polymers, amide-based polymers andmelamine-based polymers (in which the monomers to constitute acrylicpolymers do not include methacrylic acid, methyl methacrylate, acrylicacid and methyl acrylate).

Preferably, the alkylene glycol-based polymers, the cellulosicderivatives, the vinylic polymers, the acrylic polymers, the urea-basedpolymers, the epoxy polymers, the amide-based polymers and themelamine-based polymers (in which the monomers constituting acrylicpolymers do not include methacrylic acid, methyl methacrylate, acrylicacid and methyl acrylate) are those comprising any of theabove-mentioned constitutive monomers.

In the copolymer of methacrylic acid and/or methyl methacrylate with themonomer constituting the above-mentioned polymers, the monomerpreferably accounts for 5-35 mass %, more preferably 10-25 mass % ofmethacrylic acid and/or methyl methacrylate. Also preferably, thecontent of acrylic acid and/or methyl acrylate in the water-solublecopolymer is 35-75 mass %, more preferably 50-70 mass %.

(iv) The water-soluble polymer is a copolymer or a mixed resin ofpolymethacrylic acid and/or polymethyl methacrylate, polyacrylic acidand/or polymethyl acrylate, and at least one polymer selected fromalkylene glycol-based polymers, cellulosic derivatives, vinylicpolymers, acrylic polymers (excluding polymethacrylic acid, polymethylmethacrylate, polyacrylic acid and polymethyl acrylate), urea-basedpolymers, epoxy polymers, amide-based polymers and melamine-basedpolymers.

Preferably, the alkylene glycol-based polymers, the cellulosicderivatives, the vinylic polymers, the acrylic polymers (excludingpolymethacrylic acid, polymethyl methacrylate, polyacrylic acid andpolymethyl acrylate), the urea-based polymers, the epoxy polymers, theamide-based polymers and the melamine-based polymers are thosecomprising any of the above-mentioned constitutive monomers.

Preferably, the content of polymethacrylic acid and/or polymethylmethacrylate in the water-soluble polymer is 5-35 mass %, morepreferably 10-25 mass %. Also preferably, the content of polyacrylicacid and/or polymethyl acrylate in the water-soluble polymer is 35-75mass %, more preferably 50-70 mass %.

Comprising the water-soluble polymer of the above-mentioned embodiments(i) to (iv), the over-coating agent for forming fine-line patterns ofthe invention carries the advantages that the exposure margin is largeand the dimension control of photoresist patterns can be reflected onthe dimension controllability of fine-line patterns, and therefore itenjoys the best effects of invention in that the dimension control andplanning of fine-line patterns which is obtained after treatment forthermal shrinkage can be attained with ease in the stage of photoresistpatterning; that the original photoresist pattern profile can be kept assuch and the top of the photoresist pattern is not rounded after thermalshrinkage; that the degree of thermal shrinkage of the over-coatingagent relative to the temperature at which the over-coating agent isheated can be increased more than that in conventional technology; andthat the intended fine-line patterns can be formed more efficiently.

In particular, the embodiments (iii) and (iv) comprising(poly)methacrylic acid (ester) and additionally (poly)acrylic acid(ester) are especially preferred as compared with the embodiments (i)and (ii), since the exposure margin can be broadened more while thedegree of thermal shrinkage can be kept on the same level, and sincethey are more effective for smoothing the profile of the side wall ofpattern lines (smoothing effect).

In the embodiments (i) to (iv), the polymer to be selected preferablycomprises at lease one polymer selected from alkylene glycol-basedpolymers, cellulosic derivatives, vinylic polymers and acrylic polymers.One or more additional polymers may be in the copolymer. Preferredexamples of the additional polymers in the invention arepolyvinylpyrrolidone, polyvinylimidazole and acryloylmorpholine.

The over-coating agent for forming fine patters may additionally containwater-soluble amines. For special purposes such as preventing thegeneration of impurities and pH adjustment, water-soluble amines thathave pKa (acid dissociation constant) values of 7.5-13 in aqueoussolution at 25° C. are preferably used. Specific examples include thefollowing: alkanolamines, such as monoethanolamine, diethanolamine,triethanolamine, 2-(2-aminoethoxy)ethanol, N,N-dimethylethanolamine,N,N-diethylethanolamine, N,N-dibutylethanolamine, N-methylethanolamine,N-ethylethanolamine, N-butylethanolamine, N-methyldiethanolamine,monoisopropanolamine, diisopropanolamine and triisopropanolamine;polyalkylenepolyamines, such as diethylenetriamine,triethylenetetramine, propylenediamine, N,N-diethylethylenediamine,1,4-butanediamine, N-ethyl-ethylenediamine, 1,2-propanediamine,1,3-propanediamine and 1,6-hexanediamine; aliphatic amines, such astriethylamine, 2-ethyl-hexylamine, dioctylamine, tributylamine,tripropylamine, triallylamine, heptylamine and cyclohexylamine; aromaticamines, such as benzylamine and diphenylamine; and cyclic amines, suchas piperazine, N-methyl-piperazine and hydroxyethylpiperazine. Preferredones are aliphatic amines in view of easiness of keeping shapes ofpatterns, as exemplified by triethylamine.

If the water-soluble amine is to be added, it is preferably incorporatedin an amount of about 0.1-30 mass %, more preferably about 2-15 mass %,of the over-coating agent (in terms of solids content). If thewater-soluble amine is incorporated in an amount of less than 0.1 mass%, the coating fluid may deteriorate over time. If the water-solubleamine is incorporated in an amount exceeding 30 mass %, the photoresistpattern being formed may deteriorate in shape.

For such purposes as reducing the dimensions of patterns and controllingthe occurrence of defects, the over-coating agent for forming finepatterns may further optionally contain non-amine based, water-solubleorganic solvents.

As such non-amine based, water-soluble organic solvents, any non-aminebased organic solvents that can mix with water may be employed and theymay be exemplified by the following: sulfoxides, such as dimethylsulfoxide; sulfones, such as dimethylsulfone, diethylsulfone,bis(2-hydroxyethyl)sulfone and tetramethylenesulfone; amides, such asN,N-dimethylformamide, N-methylformamide, N,N-dimethylacetamide,N-methylacetamine and N,N-diethylacetamide; lactams, such asN-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N-propyl-2-pyrrolidone,N-hydroxymethyl-2-pyrrolidone and N-hydroxyethyl-2-pyrrolidone;imidazolidinones, such as 1,3-dimethyl-2-imidazolidinone,1,3-diethyl-2-imidazolidinone and 1,3-diisopropyl-2-imidazolidinone; andpolyhydric alcohols and derivatives thereof, such as ethylene glycol,ethylene glycol monomethyl ether, ethylene glycol monoethyl ether,ethylene glycol monobuthyl ether, ethylene glycol monomethyl etheracetate, ethylene glycol monoethyl ether acetate, diethylene glycol,diethylene glycol monomethyl ether, diethylene glycol monoethyl ether,diethylene glycol monobuthyl ether, propylene glycol, propylene glycolmonomethyl ether, glycerol, 1,2-butylene glycol, 1,3-butylene glycol and2,3-butylene glycol. Among those mentioned above, polyhydric alcoholsand their derivatives are preferred for the purposes of reducing thedimensions of patterns and controlling the occurrence of defects andglycerol is particularly preferred. The non-amine based, water-solubleorganic solvents may be used either singly or in combination.

If the non-amine based, water-soluble organic solvent is to be added, itis preferably incorporated in an amount of about 0.1-30 mass %, morepreferably about 0.5-15 mass %, of the water-soluble polymer. If thenon-amine based, water-soluble organic solvent is incorporated in anamount of less than 0.1 mass %, its defect reducing effect tends todecrease. Beyond 30 mass %, a mixing layer is liable to form at theinterface with the photoresist pattern.

In addition, the over-coating agent may optionally contain a surfactantfor attaining special effects such as coating uniformity and wafer'sin-plane uniformity.

The surfactant is preferably employed that, when added to thewater-soluble polymer, exhibits certain characteristics such as highsolubility, non-formation of a suspension and miscibility with thepolymer component. By using surfactants that satisfy thesecharacteristics, the occurrence of defects can be effectively controlledthat is considered to be pertinent to forming fine-line patters uponcoating the over-coating agent.

From the points above, surfactants in the invention are preferablyemployed at least the one selected among N-alkylpyrrolidones, quaternaryammonium salts and phosphate esters of polyoxyethylene.

N-alkylpyrrolidones as surfactant are preferably represented by thefollowing general formula (I):

where R₁ is an alkyl group having at least 6 carbon atoms.

Specific examples of N-alkylpyrrolidones as surfactant includeN-hexyl-2-pyrrolidone, N-heptyl-2-pyrrolidone, N-octyl-2-pyrrolidone,N-nonyl-2-pyrrolidone, N-decyl-2-pyrrolidone, N-undecyl-2-pyrrolidone,N-dodecyl-2-pyrrolidone, N-tridecyl-2-pyrrolidone,N-tetradecyl-2-pyrrolidone, N-pentadecyl-2-pyrrolidone,N-hexadecyl-2-pyrrolidone, N-heptadecyl-2-pyrrolidone andN-octadecyl-2-pyrrolidone. Among these, N-octyl-2-pyrrolidone(“SURFADONE LP 100” of ISP Inc.) is preferably used.

Quaternary ammonium salts as surfactant are preferably represented bythe following general formula (II):

where R₂, R₃, R₄ and R₅ are each independently an alkyl group or ahydroxyalkyl group (provided that at least one of them is an alkyl orhydroxyalkyl group having not less than 6 carbon atoms); X⁻ is ahydroxide ion or a halogenide ion.

Specific examples of quaternary ammonium salts as surfactant includedodecyltrimethylammonium hydroxide, tridecyltrimethylammonium hydroxide,tetradecyltrimethylammonium hydroxide, pentadecyltrimethylammoniumhydroxide, hexadecyltrimethylammonium hydroxide,heptadecyltrimethylammonium hydroxide and octadecyltrimethylammoniumhydroxide. Among these, hexadecyltrimethylammonium hydroxide ispreferably used.

Phosphate esters of polyoxyethylene are preferably represented by thefollowing general formula (III):

where R₆ is an alkyl or alkylaryl group having 1-10 carbon atoms; R₇ isa hydrogen atom or (CH₂CH₂O)R₆ (where R₆ is as defined above); n is aninteger of 1-20.

To mention specific examples, phosphate esters of polyoxyethylene thatcan be used as surfactants are commercially available under trade names“PLYSURF A212E” and “PLYSURF A210G” from Dai-ichi Kogyo Seiyaku Co.,Ltd.

If the surfactant is to be added, it is preferably incorporated in anamount of about 0.1-10 mass %, more preferably about 0.2-2 mass %, ofthe over-coating agent (in terms of solids content). By adopting theamount as described above ranges, it may effectively prevent thevariations in the percent shrinkage of patterns, potentially dependingon the wafer's in-plane uniformity which is caused by the deteriorationof coating property, and also prevent the occurrence of defects that areconsidered to have cause-and-effect relations with microfoaming on theapplied film that generates as the coating conditions are worsened.

The over-coating agent of the invention for forming fine patterns ispreferably used as an aqueous solution at a concentration of 3-50 mass%, more preferably at 5-30 mass %. If the concentration of the aqueoussolution is less than 3 mass %, poor coverage of the substrate mayresult. If the concentration of the aqueous solution exceeds 50 mass %,there is no appreciable improvement in the intended effect thatjustifies the increased concentration and the solution cannot be handledefficiently.

As already mentioned, the over-coating agent of the invention forforming fine patterns is usually employed as an aqueous solution usingwater as the solvent. A mixed solvent system comprising water and analcoholic solvent may also be employed. Exemplary alcoholic solvents aremonohydric alcohols including methyl alcohol, ethyl alcohol, propylalcohol and isopropyl alcohol. These alcoholic solvents are mixed withwater in amounts not exceeding about 30 mass %.

The over-coating agent for forming fine patterns of the inventioncarries the advantages that the exposure margin is large and thedimension control of photoresist patterns can be reflected on thedimension control of forming fine-line patterns, that the photoresistpattern profile after thermal shrinkage stage can be kept rectangularand the top thereof is prevented from being rounded, and that the degreeof thermal shrinkage of the over-coating agent is large relative to thetemperature at which the agent is heated.

The method of forming fine-line patterns according to the second aspectof the invention comprises the steps of covering a substrate havingphotoresist patterns thereon with the above-described over-coating agentfor forming fine patterns, then applying heat treatment to shrink theapplied over-coating agent under the action of heat so that the spacingbetween adjacent photoresist patterns is reduced, and subsequentlyremoving the applied film of the over-coating agent substantiallycompletely.

The method of preparing the substrate having photoresist patternsthereon is not limited to any particular type and it can be prepared byconventional methods employed in the fabrication of semiconductordevices, liquid-crystal display devices, magnetic heads and microlensarrays. In an exemplary method, a photoresist composition of chemicallyamplifiable or other type is spin- or otherwise coated on a substratesuch as a silicon wafer and dried to form a photoresist layer, which isilluminated with an activating radiation such as ultraviolet,deep-ultraviolet or excimer laser light through a desired mask patternusing a reduction-projection exposure system or subjected to electronbeam photolithography, then heated and developed with a developer suchas an alkaline aqueous solution, typically a 1-10 mass %tetramethylammonium hydroxide (TMAH) aqueous solution, thereby forming aphotoresist pattern on the substrate.

The photoresist composition serving as a material from which photoresistpatterns are formed is not limited in any particular way and any commonphotoresist compositions may be employed including those for exposure toi- or g-lines, those for exposure with an excimer laser (e.g. KrF, ArFor F₂) and those for exposure to EB (electron beams). According to theinvention, even in a photoresist having a low thermal flow temperatureof around 150° C. or lower, the top of the photoresist pattern can beprevented from being rounded, and while the degree of thermal shrinkageof the over-coating agent is kept as such, the exposure margin may bebroadened. Still another advantage of the invention is that thecontrollability in patterning is good.

[a.] Over-Coating Agent Application Step

After thusly forming the photoresist pattern as a mask pattern, theover-coating agent for forming fine patterns is applied to coverentirely the substrate. After applying the over-coating agent, thesubstrate may optionally be pre-baked at a temperature of 80-100° C. for30-90 seconds.

The over-coating agent may be applied by any methods commonly employedin the conventional heat flow process. Specifically, an aqueous solutionof the over-coating agent for forming fine patterns is applied to thesubstrate by any known application methods including bar coating, rollcoating and whirl coating with a spinner.

[b.] Heat Treatment (Thermal Shrinkage) Step

In the next step, heat treatment is performed to cause thermal shrinkageof the film of the over-coating agent. Under the resulting force ofthermal shrinkage of the film, the dimensions of the photoresist patternin contact with the film will increase by an amount equivalent to thethermal shrinkage of the film and, as the result, the photoresistpattern widens and accordingly the spacing between adjacent photoresistpatterns lessens. The spacing between adjacent photoresist patternsdetermines the diameter or width of the pattern elements to be finallyobtained, so the decrease in the spacing between adjacent photoresistpatterns contributes to reducing the diameter of each element of a holepattern or the width of each element of a trench pattern, eventuallyleading to the definition of a pattern with smaller feature sizes.

The heating temperature is not limited to any particular value as longas it is high enough to cause thermal shrinkage of the film of theover-coating agent and form or define a fine pattern. Heating ispreferably done at a temperature that will not cause thermal fluidizingof the photoresist pattern. The temperature that will not cause thermalfluidizing of the photoresist pattern is such a temperature that when asubstrate on which the photoresist pattern has been formed but no filmof the over-coating agent has been formed is heated, the photoresistpattern will not experience any dimensional changes. Performing a heattreatment under such temperature conditions is very effective forvarious reasons, e.g. a fine-line pattern of good profile can be formedmore efficiently and the duty ratio in the plane of a wafer, or thedependency on the spacing between photoresist patterns in the plane of awafer, can be reduced. Considering the softening points of a variety ofphotoresist compositions employed in current photolithographictechniques, the preferred heat treatment is usually performed within atemperature range of about 80-160° C. for 30-90 seconds, provided thatthe temperature is not high enough to cause thermal fluidizing of thephotoresist.

[c.] Over-Coating Agent Removal Step

In the subsequent step, the remaining film of the over-coating agent onthe patterns is removed by washing with an aqueous solvent, preferablypure water, for 10-60 seconds. Prior to washing with water, rinsing mayoptionally be performed with an aqueous solution of alkali (e.g.tetramethylammonium hydroxide (TMAH) or choline). The over-coating agentof the present invention is easy to remove by washing with water and itcan be completely removed from the substrate and the photoresistpattern.

As a result, each pattern on the substrate has a smaller feature sizebecause each pattern is defined by the narrowed spacing between theadjacent widened photoresist patterns.

The fine-line pattern thus formed using the over-coating agent of thepresent invention has a pattern size smaller than the resolution limitattainable by the conventional methods. In addition, it has a goodenough profile and physical properties that can fully satisfy thecharacteristics required of semiconductor devices.

Steps [a.]-[c.] may be repeated several times. By repeating steps[a.]-[c.] several times, the photoresist trace patterns (mask patterns)can be progressively widened.

The technical field of the present invention is not limited to thesemiconductor industry and it can be employed in a wide range ofapplications including the fabrication of liquid-crystal displaydevices, the production of magnetic heads and even the manufacture ofmicrolens arrays.

EXAMPLES

The following examples are provided for further illustrating the presentinvention but are in no way to be taken as limiting. Unless otherwisenoted, all amounts of ingredients are expressed in mass %.

Example 1

A copolymer of methacrylic acid and vinylpyrrolidone (2 g;polymerization ratio=9:1), triethylamine (0.12 g) and a polyoxyethyelenephosphate ester surfactant (0.02 g; “PLYSURF A210G”, product of Dai-ichiKogyo Seiyaku Co, Ltd.) were dissolved in water (27 g) to prepare anover-coating agent.

A substrate was whirl coated with a positive-acting photoresistTARF-P7052 (product of Tokyo Ohka Kogyo Co., Ltd.) and baked at 150° C.for 90 seconds to form a photoresist layer in a thickness of 0.34 □m.

The photoresist layer was exposed with an exposure unit (NSR-S302,product of Nikon Corp.), subjected to heat treatment at 100° C. for 90seconds and developed with an aqueous solution of 2.38 mass % TMAH(tetramethylammonium hydroxide) to form photoresist patterns whichdefined hole patterns with an each diameter of 140.2 nm (i.e., thespacing between the photoresist patterns, or the initial hole dimension,was 140.2 nm).

The previously prepared over-coating agent was applied onto thesubstrate including hole patterns and subjected to heat treatment at155° C. for 60 seconds, thereby reducing the each size of the holepatterns. Subsequently, the substrate was brought into contact with purewater at 23° C. to remove the over-coating agent. The each diameter ofthe hole patterns was reduced to 120.5 nm. The photoresist patternprofile was kept rectangular and its top was prevented from beingrounded.

Example 2

A copolymer of methacrylic acid and vinylpyrrolidone (1 g;polymerization ratio=9:1), a copolymer of acrylic acid andvinylpyrrolidone (1 g; polymerization ratio=2:1), triethylamine (0.12 g)and a polyoxyethyelene phosphate ester surfactant (0.02 g; “PLYSURFA210G”, product of Dai-ichi Kogyo Seiyaku Co, Ltd.) were dissolved inwater (27 g) to prepare an over-coating agent.

A substrate was whirl coated with a positive-acting photoresistTARF-P7052 (product of Tokyo Ohka Kogyo Co., Ltd.) and baked at 115° C.for 90 seconds to form a photoresist layer in a thickness of 0.34 □m.

The photoresist layer was exposed with an exposure unit (NSR-S302,product of Nikon Corp.), subjected to heat treatment at 100° C. for 90seconds and developed with an aqueous solution of 2.38 mass % TMAH(tetramethylammonium hydroxide) to form photoresist patterns whichdefined hole patterns with an each diameter of 140.2 nm (i.e., thespacing between the photoresist patterns, or the initial hole dimension,was 140.2 nm).

The previously prepared over-coating agent was applied onto thesubstrate including hole patterns and subjected to heat treatment at155° C. for 60 seconds, thereby reducing the each size of the holepatterns. Subsequently, the substrate was brought into contact with purewater at 23° C. to remove the over-coating agent. The each diameter ofthe hole patterns was reduced to 119.7 nm. The photoresist patternprofile was kept rectangular and its top was prevented from beingrounded.

Example 3

A copolymer of methacrylic acid, acrylic acid and vinylpyrrolidone (2 g;polymerization ratio=17:60:23), triethylamine (0.12 g) and apolyoxyethyelene phosphate ester surfactant (0.02 g; “PLYSURF A210G”,product of Dai-ichi Kogyo Seiyaku Co, Ltd.) were dissolved in water (27g) to prepare an over-coating agent.

A substrate was whirl coated with a positive-acting photoresistTARF-P7052 (product of Tokyo Ohka Kogyo Co., Ltd.) and baked at 115° C.for 90 seconds to form a photoresist layer in a thickness of 0.34 □m.

The photoresist layer was exposed with an exposure unit (NSR-S302,product of Nikon Corp.), subjected to heat treatment at 100° C. for 90seconds and developed with an aqueous solution of 2.38 mass % TMAH(tetramethylammonium hydroxide) to form photoresist patterns whichdefined hole patterns with an each diameter of 140.2 nm (i.e., thespacing between the photoresist patterns, or the initial hole dimension,was 140.2 nm).

The previously prepared over-coating agent was applied onto thesubstrate including hole patterns and subjected to heat treatment at155° C. for 60 seconds, thereby reducing the each size of the holepatterns. Subsequently, the substrate was brought into contact with purewater at 23° C. to remove the over-coating agent. The each diameter ofthe hole patterns was reduced to 119.5 nm. The photoresist patternprofile was kept rectangular and its top was prevented from beingrounded.

Comparative Example 1

A copolymer of acrylic acid and vinylpyrrolidone (2 g; polymerizationratio=2:1), triethylamine (0.12 g) and a polyoxyethyelene phosphateester surfactant (0.02 g; “PLYSURF A210G”, product of Dai-ichi KogyoSeiyaku Co, Ltd.) were dissolved in water (27 g) to prepare anover-coating agent.

The thusly prepared over-coating agent was then applied onto thesubstrate including hole patterns that were formed in the same manner asdescribed in Example 1 (the initial hole dimension, was 140.2 nm), andsubjected to heat treatment at 155° C. for 60 seconds, thereby reducingthe each size of the hole patterns. Subsequently, the substrate wasbrought into contact with pure water at 23° C. to remove theover-coating agent. The each diameter of the hole patterns was reducedto 119.9 nm, however the top of the photoresist pattern was rounded.

As described in detail hereinabove, the over-coating agent for formingfine patterns of the invention carries the advantages that the exposuremargin is large and the dimension control of photoresist patterns can bereflected on the dimension control of forming fine-line patterns, thatthe photoresist pattern profile after thermal shrinkage stage can bekept rectangular and the top thereof is prevented from being rounded,and that the degree of thermal shrinkage of the over-coating agent islarge relative to the temperature at which the agent is heated.

1. An over-coating agent for forming fine patterns which is applied tocover a substrate having photoresist patterns thereon and allowed toshrink under heat so that the spacing between adjacent photoresistpatterns is lessened, further characterized by comprising awater-soluble polymer which contains at least methacrylic acid and/ormethyl methacrylate as the constitutive monomer thereof.
 2. Theover-coating agent for forming fine patterns according to claim 1,further comprising an aliphatic amine.
 3. The over-coating agent forforming fine patterns according to claim 2, wherein the aliphatic amineis triethylamine.
 4. The over-coating agent for forming fine patternsaccording to claim 1, wherein the water-soluble polymer is a copolymerof methacrylic acid and/or methyl methacrylate with at least one monomerselected from those constituting alkylene glycol-based polymers,cellulosic derivatives, vinylic polymers, acrylic polymers, urea-basedpolymers, epoxy polymers, amide-based polymers and melamine-basedpolymers (in which the monomers to constitute acrylic polymers do notinclude methacrylic acid and methyl methacrylate).
 5. The over-coatingagent for forming fine patterns according to claim 4, wherein thewater-soluble polymer contains methacrylic acid and/or methylmethacrylate in a ratio of 60-99 mass % of the polymer.
 6. Theover-coating agent for forming fine patterns according to claim 1,wherein the water-soluble polymer is a copolymer or a mixed resin ofpolymethacrylic acid and/or polymethyl methacrylate with at least onepolymer selected from alkylene glycol-based polymers, cellulosicderivatives, vinylic polymers, acrylic polymers (excludingpolymethacrylic acid and polymethyl methacrylate), urea-based polymers,epoxy polymers, amide-based polymers and melamine-based polymers.
 7. Theover-coating agent for forming fine patterns according to claim 6,wherein the water-soluble polymer contains polymethacrylic acid and/orpolymethyl methacrylate in a ratio of 60-99 mass % of the polymer. 8.The over-coating agent for forming fine patterns according to claim 1,wherein the water-soluble polymer is a copolymer of methacrylic acidand/or methyl methacrylate, acrylic acid and/or methyl acrylate, and atleast one monomer selected from polymers constituting alkyleneglycol-based polymers, cellulosic derivatives, vinylic polymers, acrylicpolymers, urea-based polymers, epoxy polymers, amide-based polymers andmelamine-based polymers (in which the monomers to constitute acrylicpolymers do not include methacrylic acid, methyl methacrylate, acrylicacid and methyl acrylate).
 9. The over-coating agent for forming finepatterns according to claim 8, wherein the water-soluble polymercontains methacrylic acid and/or methyl methacrylate in a ratio of 5-35mass % of the polymer, and acrylic acid and/or methyl acrylate in aratio of 35-75 mass % of the polymer.
 10. The over-coating agent forforming fine patterns according to claim 1, wherein the water-solublepolymer is a copolymer or a mixed resin of polymethacrylic acid and/orpolymethyl methacrylate, polyacrylic acid and/or polymethyl acrylate,and at least one polymer selected from alkylene glycol-based polymers,cellulosic derivatives, vinylic polymers, acrylic polymers (notincluding polymethacrylic acid, polymethyl methacrylate, polyacrylicacid and polymethyl acrylate), urea-based polymers, epoxy polymers,amide-based polymers and melamine-based polymers.
 11. The over-coatingagent for forming fine patterns according to claim 10, wherein thewater-soluble polymer contains polymethacrylic acid and/or polymethylmethacrylate in a ratio of 5-35 mass % of the polymer, and polyacrylicacid and/or polymethyl acrylate in a ratio of 35-75 mass % of thepolymer.
 12. The over-coating agent for forming fine patterns accordingto claim 1, which is an aqueous solution having a concentration of 3-50mass %.
 13. A method of forming fine patterns comprising the steps ofcovering a substrate having thereon photoresist patterns with theover-coating agent for forming fine patterns according to claim 1, thenapplying heat treatment to shrink the applied over-coating agent underthe action of heat so that the spacing between adjacent photoresistpatterns is lessened, and subsequently removing the applied film of theover-coating agent substantially completely.
 14. The method of formingfine patterns according to claim 13, wherein the heat treatment isperformed by heating the substrate at a temperature that does not causethermal fluidizing of the photoresist patterns on the substrate.